Datasheet

LTC6244
18
6244fb
Figure 5b: Output Noise Spectral Density of Figure 5a. The
Simple JFET Bootstrap Improves Noise (and Bandwidth)
Drastically. Noise Density at 10kHz is Now 220nV/√Hz, About
a 8.2x Reduction. This is Mostly Due to the Bootstrap Effect
of Swapping the 1nV/√Hz of the JFET for the 7.8nV/√Hz of the
Op Amp
Figure 5a. Large Area Diode Bootstrapping
Figure 6b: Output Spectrum of Circuit of Figure 6a, with
Photodiode Bias at 0V. Photodiode Capacitance is Back Up,
as in the Original Circuit of Figure 4a. However, it can be
Reduced Arbitrarily by Providing Reverse Bias. This Plot
Shows that Bootstrapping Alone Reduced the 10kHz Noise
Density by a Factor of 6.2, from 1800nV/√Hz to 291nV/√Hz
Figure 6a. The Addition of a Capacitor and Resistor Enable the
Benefi t of Bootstrapping While Applying Arbitrary Photodiode
Bias Voltage V
BB
by providing reverse bias, and the photodiode can also be
reversed to support either cathode or anode connections
for positive or negative going outputs.
The circuit on the last page of this data sheet shows fur-
ther reduction in noise by paralleling four JFETs to attain
152nV/√Hz at 10kHz, a noise of 12 times less than the
basic photodiode circuit of Figure 4a.
The solution is as shown in the circuit of Figure 6a, which
uses a capacitor-resistor pair to enable the AC benefi ts of
bootstrapping while allowing a different reverse DC voltage
on the photodiode. The JFET is still running at the same
current, but an arbitrary reverse bias may be applied to
the photodiode. The output noise spectrum of the circuit
with 0V of photodiode reverse bias is shown in Figure 6b.
Photodiode capacitance is again 3650pF, as in the original
circuit of Figure 4a. This noise plot with 0V bias shows
that bootstrapping alone was responsible for a factor of
6.2 noise reduction, from 1800nV/√Hz to 291nV/√Hz at
10kHz, independent of photodiode capacitance. However,
photodiode capacitance can now can be reduced arbitrarily
APPLICATIONS INFORMATION
5V
V
OUT
= 1M • I
PD
BW = 350kHz
OUTPUT NOISE = 220nV/√Hz
AT10kHz
I
PD
R
F
1M
PHILIPS
BF862
JFET
R
BIAS
4.99k
HAMAMATSU
LARGE AREA
PHOTODIODE
S1227-1010BQ
C
PD
= 3000pF
C
F
0.25pF
–5V
–5V
5V
6244 F04a
–
+
1/2
LTC6244HV
V
OUT
5V
V
OUT
= 1M • I
PD
BW = 250kHz
OUTPUT NOISE = 291nV/√Hz
AT 10kHz
5V
V
BB
I
PD
R
F
1M
PHILIPS
BF862
JFET
HAMAMATSU LARGE AREA
PHOTODIODE
S1227-1010BQ
C
PD
= 3000pF
C
F
0.25pF
–5V
6244 F06a
–
+
1/2
LTC6244HV
–5V
4.99k
4.7µF
X7R
4.99k
V
OUT
1k 10k
FREQUENCY (Hz)
6244 F05b
100k
OUTPUT NOISE (200nV/√Hz/DIV)
1k 10k
FREQUENCY (Hz)
6244 F06b
100k
OUTPUT NOISE (275nV/√Hz/DIV)